This invention relates to a method and an apparatus for measuring freeness of paper stock to be fed to a paper machine during paper making process. This freeness indicates the rate of drainage of the stock on wire cloth of the paper machine. The method and apparatus makes it possible to automatically and continuously measure the freeness of the stock on the way to the paper machine or at a place where the stock is prepared.
Previously, paper stock has been treated by a beater or other similar refining equipment. The extent of beating has been measured by taking out a test sample of the stock from the beater and measuring the freeness thereof by a conventional freeness tester such as a Schopper-Riegler or Canadian Standard type freeness tester; the beating has been continued until the desired degree of beating is obtained. Recently, the refining process has been improved so that it can be performed continuously and automatically; furthermore, refined stock is sent to machine room directly and automatically. Thus, it has become necessary to measure the freeness automatically during the process, for example by taking out a sample from a feeding pipe connected to the paper machine room or from stock chest at an intermediate portion of the feeding pipe. Thus the freeness measurement of the stock in the continuous flowline now employs the steps of periodically sampling from the feeding pipe an adequate amount of sample, screening the sample and measuring the filtrate or screened water, by volume or weight, which passes through a screen during a predetermined time (usually 10 to 60 seconds). The quantity of filtrate is the index of freeness.
However, in the conventional measuring system mentioned just above, the value of the freeness varies according to variance in consistency and/or temperature of the stock. Therefore, the consistency and the temperature of the stock must be inspected from time to time and the obtained value of the freeness must be adjusted for these factors. Furthermore, according to such a measuring system, accurate measurement of the freeness of any paper stock which has been refined or beaten to a considerably high degree is impossible, since in such stock the difference in freeness is hardly detectable even if the degree of refining or beating treatment of the stock is changed.
A similar method is also applied in the feeding pipe by introducing the paper stock into a freeness tester by means of fluid pressure in the feeding pipe, and measuring the quantity of filtrate passing through a screen during a predetermined time. In such a way, the filtrate must be discharged and the fibrous mat of the paper stock left on the screen must be removed before the next test begins. However, the filtrate contains fine fibers and size, and counter flow cleaning causes the fine fibers and size to stick to the back side of the screen. Such a phenomenon is likely to cause error in the next test. Therefore, the screen must be dismounted and cleaned from time to time and the consistency of the stock in the feeding system is varied because of white water flow into the feeding pipe.